I asked HV who said she would look into it, but never got back to me..... and e-mailed Social Services Families/Disabilities team and no response.
DS is getting to the age (reception age) where I find parents aren't always welcome at classes, but DS's language problems mean that he's not going to understand instructions all that well. I'ld love to find something like a football class or trampoline class or even possibly martial arts class that would be understanding to his language problems.